Make `toggle-files-button` feature persistent across pages #3588

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Currently the toggle-file-button feature is not extremely useful, as it basically gets you to the readme quicker, and that's it. Ever since a new link popped up on the right side of the page, it might make sense to remove this feature as mentioned in #3580 and #3587.

But I think it would be very nice to keep this feature, and to give more usefulness by making it persist during navigation -- meaning if the file list is toggled off, it will be off when reloading or navigating to a different repo page.

I seldom browse the repo files directly on GitHub, so having the list hidden by default would save quite a bit of scrolling (and yes I do know and use "Hide Files on GitHub", but it doesn't really help with this kind of repo for example).

Feature checklist:

1. It doesn't already exist as a userscript or extension.
2. It makes sense as a default for everyone: if you hide the files on a repo page, there's a good chance you might want it hidden on the other pages as well. Otherwise if you just want to jump to the readme, there's a link for that.
3. It doesn't require options: if the list is open, it stays open ; if it's closed it stays closed.
4. It doesn't require a lot of view/UI, code and effort to maintain: no new UI, but the business logic might be a tad tricky (I tried a naive implementation by adding a check in init() but it creates some annoying jitter on page load)
5. It doesn't require an external server.
6. It doesn't require too much HTTP traffic.
7. It doesn't enhance "Projects"

I'm willing to try implementing this if you accept it as a valid feature request.

Thank you for maintaining and improving this indispensable extension! 👍